Induction heating device and method for metallic implants in living beings

ABSTRACT

A system and method for heating cells surrounding metallic implants in living beings. The system includes a metallic implant within the living being, surrounded by the cells to be heated, an induction coil having an aperture sufficiently large to accommodate a portion of the living being containing the metallic implant, and an RF generator coupled to the coil to apply an electric RF signal thereto, the RF signal ranging in frequency from 10 kilohertz to 10 megahertz. The metallic implant is inductively heatable by the application of the RF signal to the induction coil.

FIELD OF THE INVENTION

The present invention is directed to a device and method for use withmetal implants in living beings to heat cells immediately surrounding ametallic implant. The device and method can be used, for example, fortreatment of hyperplastic cell growth conditions.

BACKGROUND OF THE INVENTION

Metallic implants are successfully used inside living beings for manypurposes. For example, stents, or tubular endoprosthesises, are used inthe human vascular system to widen or prevent further narrowing, orstenosis, caused by arteriosclerosis. A stent is often placed in avessel after an angioplasty. Angioplasty is a known procedure forwidening a vessel, but may cause trauma resulting in longitudinalruptures in the intima. Application of a stent to the region of theangioplasty provides a continuous widening force on the vessel wall.However, the damaged blood vessel can react by generating cells torepair itself. In some cases, excessive cell generation, termed “intimalhyperplasia,” occurs at the intima surrounding the stent. Intimalhyperplasia is the abnormal multiplication of normal cells in anabnormal cell arrangement in the tissue. Hyperplasia may result in are-narrowing of the blood vessel, or restenosis.

Metallic implants can also be used to palliatively treat the narrowingof hollow organs caused by tumors. For example, tumors near the trachea,esophagus, biliary ducts, or urinary tract can press upon and narrowthese organs. A stent can be placed in the organ to preserve its shapeand function. A stent is typically constructed of frame of material,called a stent matrix, which includes a pattern of interstices. Thetumor may eventually grow into the stent, through these interstices,thus causing narrowing of the stented hollow organ. As a result,continued growth of the tumor may overcome the palliative effect of thestent.

Aneurysms, internal bleeding of vessels, leakages of vessels, andtumorous vessels can be occluded by metallic implants, such as metallicspirals or stents, put in over catheters to seal the vessel leak.However, complete occlusion is not always achieved because blood leakagemay continue through spaces in the spiral or stent matrix following theplacement of the stent in the vessel. Therefore, the aneurysm, forexample, may continue to grow despite the presence of the implant, andthe bleeding may not be completely stopped. Currently, stents coveredwith DACRON® material are often used to seal an aneurysm. However, thestents covered with DACRON® material are thick and therefore require alarge introduction site and may stretch the vessel wall considerably.

Therefore, a device and treatment method are needed which reduce theexcessive tissue reaction, for example, in restenosis followingangioplasty. There is also a need for a device and treatment method forreducing the excrescence of tumor cells surrounding metallic implants.Further, a device and treatment method are needed to complete a vascularocclusion by metallic implants without invasive procedures.

SUMMARY OF THE INVENTION

The invention provides a system for heating cells surrounding a metallicimplant implanted in a living being, comprising an RF generator togenerate an RF signal. The system also includes an induction coilcoupled to the RF generator to receive the RF signal therefore, the coilhaving an aperture therethrough sufficiently wide to receive a portionof the living being so that the metallic implant is locatable within thecoil, and a controller coupled to the generator to control the RF signalapplied to the coil. According to the invention an RF induction signalat a location within the coil is operative to inductively heat ametallic implant placed within the coil.

The invention also describes a method for heating cells surrounding ametallic implant in a living being, the method comprising inductivelyheating the metallic implant to a temperature sufficient to heat thecells.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is an electrical schematic diagram of the system of the presentinvention.

FIG. 2 is a view of the system of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE VARIOUS EMBODIMENTS

The present invention claims priority to DE 295 19 982.2, the disclosureof which is incorporated herein.

The present invention is applicable to prolonging and improving themedical use of metallic implants in living beings. Metallic implants arepieces of metal that are placed within a living being. A metal isdefined as a material that is inductively heatable according to thepresent invention. A metal is an electrically conductive material. Theinvention is advantageous where excessive tissue reaction occurssurrounding metallic implants or where there is an excrescence of tumorcells surrounding metallic implants. The invention is also useful wherethere is an uncompleted vascular occlusion by metallic implants. Whilethe present invention is not so limited, an appreciation of the variousaspects of the invention will be provided through a discussion ofvarious application examples operating in such an environment. Thedevice and method disclosed are suitable for human or animal use.

By heating cells at the site of intimal hyperplasia, excessive cellgeneration in a vessel wall following angioplasty can be reduced so thatthe narrowing effects associated with vascular restenosis postangioplasty are reduced.

Further, heating cells that grow into a stent at a stented hollow organcan reduce the expansion of the tumor into that area, therebymaintaining the integrity of the passage for a longer time period.

Heating a metallic implant inside a living being raises the temperatureof the living cells that immediately surround the implant. If the risein temperature is sufficiently large, the cells will shrink and cellgeneration will slow or stop. By heating the implant inductively, thislocalized thermal cell damage of the cells in the immediate neighborhoodof the implant can be achieved without requiring an invasive procedure.Thus, the advantages discussed above can be realized using a noninvasivetechnique. A conducting coil and an RF generator are used in the presentinvention to heat a metallic implant inductively from outside the body.The metallic implant then heats the surrounding cells by conduction.Raising the cell temperature to above 45° C., or more particularly to arange of approximately 50° C. to 65° C., achieves the desired localizedcell damage according to the present invention.

FIG. 1 illustrates one embodiment of the disclosure in which aninductive coil 10 and a generator 12 are shown. The coil includesseveral windings with an electrical conductor 14, for example, copperwire or copper tubing. The coil 10 has an inner opening or aperturesufficiently large to permit a portion of the body with a metallicimplant 16, or metallic endoprosthesis, to be placed within the coil, asshown in FIG. 2. The body part having the metallic implant 16 may bepositioned approximately near the center axis of the coil 10.

An RF electric signal is applied to the coil 10, creating an alternatingmagnetic field in the space inside the coil 10. The alternating magneticfield creates changing electric fields in the implant 16. As electronsflow across regions of the metal to equalize the electric charge acrossdifferent regions, eddy currents are excited in the metal and flowwithin the metallic implant 16. The eddy currents heat the metallicimplant 16 due to resistive heating.

The amount of heat produced by the eddy currents depends on thefrequency and power of the electric signal applied to the coil andduration of exposure to the magnetic fields. Suitable materials for ametallic implant, such as a stent, include stainless steel, nickeltitanium, alloys including nickel, titanium, vanadium, and/or chromium,and other biocompatible metals. The goal of the inductive heating is toraise the temperature of the cells surrounding a stent matrix so thatthe cells will shrink and cell generation will slow. The temperature maybe raised to above 45° C. or, more particularly, to 50° C. to 65° C.,where heat damage and cell death may be induced.

A pulsed RF signal may be applied to the coil 10 at repetitiveintervals. A controller may be coupled to the generator to control theapplication of the RF signal to the coil in the desired manner. Theduration of each pulse may range from 1 millisecond to 100 seconds, moreparticularly from 10 milliseconds to 1 second. The pulse rate of the RFsignal may range from approximately 100 Hz to 0.1 Hz, more particularlyfrom 10 Hz to 0.1 Hz. In one particular example, an RF pulse lasting for100 milliseconds, may be applied five times per second. It will beappreciated that other pulse lengths and repetition rates may beemployed to achieve the desired heating. Also, the RF signal may becontinuously applied.

The RF signal, pulsed or continuous, may be applied for a duration ofapproximately 0.1 to 30 seconds or, in other embodiments, approximately5 to 20 seconds, or approximately 3 to 15 seconds. It will beappreciated that the time of application, duration of pulse, frequencyof the signal, power of the signal, and other variables may be adjustedin various combinations to achieve the desired heating of the metallicimplant.

The generator preferably produces a radio frequency signal of 10kilohertz to 10 megahertz. In one particular embodiment, the generatorapplies a signal having a frequency in the range of 30 kHz to 1 MHz.When the frequency of the RF signal is outside the range 30 kHz to 1MHz, the inductive heating effect decreases. It has been found that thetemperature of a stent does not significantly increase when subjected tothe RF field of a magnetic resonance imaging system, typically having afrequency in the range of 20 MHz to 70 MHz, varying with the magneticfield strength employed.

The peak power of the RF signal applied to the coil may range from 1 kWto 1 MW, while in one particular embodiment, the peak power ranges from5 kW to 20 kW. In another embodiment, the peak power is approximately 15kW.

The inner aperture of the coil used may range in size from 3 cm to 1 macross, more particularly from 4 cm to 60 cm. When a coil having asmaller aperture is used,. for example, of 4 cm, it is important thatthe implant to be inductively heated is positioned centrally within thecoil. In a coil with an average diameter of 4 cm, the inductive heatingeffect decreasees substantially as the metallic implant is moved awayfrom the center of the aperture. The coil will consist of a conductivematerial, and could be wire or tubing, for example. If tubing is used,water may flow through the tubing for cooling, if necessary.

Experiments have been conducted to test the efficacy of the inductioncoil in heating a metal implant. In these experiments, stents surroundedby biological tissue were used as the implants. In some cases, thetissue comprised cells cultivated on the stent matrix itself, forexample, tumor cells. In other experiments, the stent was firstimplanted in a blood vessel of an animal, and the portion of the vesselcontaining the stent was then harvested from the animal. Typically, themass of vessel tissue was approximately 20 grams.

The diameter of the stents used in these experiments typically rangedfrom about 8 to 12 mm, and from about 1 to 3 cm in length. Prior toexposure to the inductive RF field, the stent and tissue were placed inabout 10 ml of saline solution to approximate the thermally conductiveenvironment of the body.

The RF generator used in these experiments produced an RF signal havinga frequency of about 300 kHz, and at a peak power of 15 kW. Thegenerator was typically operated in a pulsed mode, producing a 100 mspulse at a repetition rate of 5 pulses per second. Thus the averagepower applied to the coil was 7.5 kW. The samples were exposed to thepulsed RF signal for a duration of between 3 and 15 seconds.

The coil used in the experiments was formed from copper tubing having adiameter of approximately 10 mm, with a wall thickness of 2 mm. The coilhad five turns of wire and had an average diameter of 4 cm. The lengthof the coil along its longitudinal axis was 7 cm. Cooling the tubing bywater was not necessary in this device.

It was found that, under the conditions described above, the temperatureof the tissue samples surrounding the stents increased to a temperatureranging from 50° C. to 65° C. Necrosis of the cells was observed afterthe temperature was raised to this range.

When the implant is heated, living cells surrounding the implant, ormetallic stent matrix, are heated, and may be thermally damaged. Theextent of the area of cells heated is at least partially regulated bythe duration of the exposure to the RF field. The temperature of thesecells is raised because heat from the selectively heated implant isconducted into the surrounding tissue. The nuclei and cytoplasm of theneighboring cells shrink as a result of the conducted heat. It will beappreciated that a variation in the temperature of the heated implantresults in a variation in the range of damage surrounding the implant.If the body of a living being is exposed in an induction heating deviceat frequency ranging from 30 kHz-1 MHz, a metallic implant is heatedselectively and there is no measurable increase in the temperature ofnormal body tissue.

The method of the present invention may be used repeatedly during acourse of fractionated therapy. The metallic implant may be inductivelyheated periodically to prevent the occurrence of restenosis, forexample, over periods of weeks or months.

The method of the invention can be used to heat cells immediatelysurrounding metallic implants for the purposes of, for example, reducingexcessive tissue reaction in the vascular system following stentimplantation.

The method may also be used for preventing the excrescence of tumorcells surrounding metallic implants.

Another application of the present invention is to complete occlusion ofa stented aneurysm when, for example, a stent is used at an aneurysmsite. Typically, a stent has a matrix of interstitial spaces in thestent wall. Blood leakage may continue through these spaces followingthe placement of the stent in the vessel. By a controlled warmingprocess of the stent according to the present invention, the formationof thrombi can be encouraged in the holes of the stent. An accumulationof thrombi and fibers over the stent blocks further leakage through theaneurysm. Therefore, where an implant is used to occlude an aneurysm andthe implant is heated, a clot may be encouraged to form at the aneurysmsite, thereby preventing further bleeding through the vessel wall. Oneconcern with this application is that small thrombi might form at theedges of the stent matrix and break free from the stent.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.For example, the term metallic implant that is used in connection withthis invention includes devices other than stents, such as embolizationcoils, metallic particles used for embolization purposes, and othermetallic devices, implants, and endoprotheses.

What is claimed is:
 1. A system for heating cells in a living being, thesystem comprising: a metallic stent for implanting within the livingbeing, the metallic stent surrounded by the cells to be heated; aninduction coil having an aperture of a size sufficiently large toaccommodate a portion of the living being containing the metallicimplant; and an RF generator coupled to the coil to apply an electric RFsignal thereto, the RF signal ranging in frequency from 10 kilohertz to10 megahertz; wherein the metallic implant is inductively heatable bythe application of the RF signal to the induction coil.
 2. The system ofclaim 1, wherein the signal has a frequency ranging from 30 kilohertz to1 megahertz.
 3. The system of claim 1, wherein the coil has an averagediameter ranging from 3 to 100 centimeters.
 4. The system of claim 1,further comprising a controller coupled to the generator to pulse the RFsignal applied to the coil.
 5. The system of claim 4, wherein a pulserate ranges from 100 Hertz to 0.1 Hertz.
 6. The system of claim 4,wherein a pulse of RF signal has a duration ranging from 1 millisecondto 100 seconds.
 7. The system of claim 1, wherein the RF signal has apeak power ranging from 1 kW to 1 MW.
 8. The system of claim 1, whereinthe RF signal has a peak power of 15 kW.
 9. The system of claim 1,further comprising a platform to support the portion of the living beingwithin the induction coil.
 10. A system for heating cells surrounding astent implanted in a living being, the system comprising: a metallicstent, the metallic stent implanted in the living being; an RF generatorto generate an RF signal having a frequency in the range ofapproximately 10 kilohertz to 10 megahertz and a peak power in the rangeof approximately 1 kW to 1 MW; an induction coil coupled to the RFgenerator to receive the RF signal therefore, the coil having anaperture therethrough having a size sufficient to receive a portion ofthe living being so that the stent is locatable within the coil; and acontroller coupled to the generator to control the RF signal applied tothe coil; wherein an RF induction signal at a location within the coilis operative to inductively heat a stent placed within the coil.
 11. Thesystem of claim 10, wherein the controller applies the RF signal to thecoil at a pulsed rate ranging from 100 Hertz to 0.1 Hertz.
 12. Thesystem of claim 10, wherein the controller applies the RF signal to thecoil at a pulsed rate, each pulse having a duration ranging from 1millisecond to 100 seconds.
 13. The system of claim 10, wherein the RFsignal has a peak power of 15 kW.
 14. The system of claim 10, furthercomprising a platform to support the portion of the living being withinthe induction coil.
 15. A system for treating intimal hyperplasia byheating cells surrounding a stent implanted in a living being, thesystem comprising: a metallic stent, the metallic stent implanted in theliving being; an RF generator to generate an RF signal having afrequency in the range of approximately 10 kilohertz to 10 megahertz anda peak power in the range of approximately 1 kW to 1 MW; an inductioncoil coupled to the RF generator to receive the RF signal therefore, thecoil having an aperture therethrough having a diameter sufficient toreceive a portion of the living being so that the stent is locatablewithin the coil; and a controller coupled to the generator to controlthe RF signal applied to the coil; wherein an RF induction signal at alocation within the coil is operative to inductively heat a stent placedwithin the coil.
 16. A method for heating cells surrounding a metallicstent implanted in a living being, the method comprising inductivelyheating the metallic stent to a temperature sufficient to heat thecells.
 17. The method of claim 16, wherein the step of inductivelyheating further comprises applying an RF signal to an inductive coilhaving an aperture therethrough, where a portion of the living beingcontaining the metallic implant is located in the aperture.
 18. Themethod of claim 17, wherein the RF signal has a frequency ranging fromapproximately 1 kilohertz to 10 megahertz.
 19. The method of claim 17,wherein the RF signal has a frequency ranging from approximately 30kilohertz to 1 megahertz.
 20. The method of claim 17, wherein theapplication of the RF signal to the coil is pulsed.
 21. The method ofclaim 20, wherein the application of the RF signal to the coil has apulse rate ranging from approximately 100 Hertz to 0.1 Hertz.
 22. Themethod of claim 20, wherein the application of the RF signal to the coilhas a pulse duration ranging from approximately 1 millisecond to 100seconds.
 23. The method of claim 17, wherein the RF signal applied tothe coil has a peak power ranging from approximately 1 kW to 1 MW.
 24. Amethod for heating cells surrounding a metallic stent in a living being,the method comprising: positioning the living being so that the metallicstent is located substantially within an aperture of an inductive coil;and applying an RF signal to the coil, so that the metallic stent isinductively heated with the living being; wherein the RF signal appliedto the coil is pulsed.
 25. The method of claim 24, wherein the RF signalapplied to the coil has a frequency ranging from approximately 1kilohertz to 10 megahertz.
 26. The method of claim 24, wherein the RFsignal applied to the coil has a frequency ranging from approximately 30kilohertz to 1 megahertz.
 27. The method of claim 24, wherein the RFsignal is applied to the coil for at least 5 seconds and not more than20 seconds.
 28. The method of claim 24, wherein the application of theRF signal to the coil has a pulse rate ranging from approximately 100Hertz to 0.1 Hertz.
 29. The method of claim 24, wherein the applicationof the RF signal to the coil has a pulse duration ranging fromapproximately 1 millisecond to 100 seconds.
 30. The method of claim 24,wherein the RF signal applied to the coil has a peak power ranging fromapproximately 1 kW to 1 MW.
 31. A method for preventing intimalhyperplasia at a site of an angioplasty, comprising: implanting ametallic implant at the site of the angioplasty; and inductively heatingthe implant to a temperature sufficient to heat the cells surroundingthe implant.
 32. The method of claim 31, further comprising repeatingthe inductive heating step at regular intervals.
 33. A method forcomplete exclusion of an aneurysm, comprising: positioning a metallicimplant defining interstitial spaces at a site of the aneurysm;inductively heating the metallic implant to a temperature sufficient toinduce formation of thrombi in the interstitial spaces of the metallicimplant.